The next generation mobile communication systems and wireless transmission systems in multi-cell environments require an enhanced data transmission rate and system capacity. Accordingly, have been performed research for multi-input multi-output (MIMO) systems capable of transmitting data by using a plurality of antennas. Among the MIMO systems, a closed loop MIMO system enhances a data transmission function by utilizing fed-back channel information so as to enhance a data transmission rate in multi-cell environments.
FIG. 1 is a view showing a multi-cellular mobile communication system with consideration of cooperative transmission between base stations.
Referring to FIG. 1, a first base station 30 communicates with a first terminal 10, and a second base station 40 communicates with a second terminal 20.
The first and second terminals 10 and 20 feedback their channel information to one or more base stations 30 and 40. Accordingly, the fed-back information is shared between the respective base stations through a controller 50. Each station transmits information to each terminal based on channel information fed-back from the terminal. This may allow the base stations to efficiently communication with the terminals.
FIG. 2 is a configuration view of a Closed Loop Multi-Input Multi-Output (MIMO) system in accordance with the conventional art.
Referring to FIG. 2, the base station 30 denotes a transmitter, and the terminal 10 denotes a receiver.
The conventional closed-loop MIMO system comprises a base station 30, a transmission antenna 31, a terminal 10, and a reception antenna 11. And, a transmission signal is transmitted to the terminal 10 after passing through an H matrix.
The base station 30 includes a coding and modulation unit 32 configured to coding and modulation processes with respect to a transmission signal, a coding book 33, and a processor 34 for multiplying one weight vector included in the coding book 33 by the transmission signal.
The terminal 10 receives a signal transmitted from the base station 30 through the reception antenna 11, thereby demodulating the received signal by selecting one coding vector from the coding book 12.
The terminal 10 feedbacks its channel information to the base station 30, thereby allowing the base station 30 to efficiently communicate with itself based on the channel information.
Methods for enhancing a data transmission function by using channel information by the closed-loop MIMO system include a non-cooperative transmission method (1) and a cooperative transmission method (2).
The non-cooperative transmission method denotes a method for transmitting information, by a single base station, to a terminal based on channel environments of the terminal within a cell coverage. And, the cooperative transmission method denotes a method for transmitting information, by a plurality of base stations, to a plurality of terminals based on channel environments of the terminals within each cell coverage.
Firstly, will be explained a Signal-to-Noise Ratio (SNR) of the closed loop MIMO system based on a non-cooperative transmission method.
In the closed loop MIMO system based on a non-cooperative transmission method, an optimized system is designed without considering channel environments of other terminals. However, in the closed loop MIMO system based on a cooperative transmission method, an optimized system is designed with consideration of channel environments of other terminals. Accordingly, the conventional system is designed in an assumption that each channel is independently and identically distributed. The conventional system has the following configuration.
NR×1 reception signal vectors received by a terminal (receiver) having NR antennas from a base station (transmitter) having NT antennas can be expressed as follows.
                    r        =                  Hx          +                                                    n                ⁡                                  [                                                                                                              h                          11                                                                                            …                                                                                              h                                                      1                            ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                    ⋱                                                                    ⋮                                                                                                                                      h                                                                                    N                              R                                                        ⁢                            1                                                                                                                      …                                                                                              h                                                                                    N                              R                                                        ⁢                                                          N                              T                                                                                                                                                            ]                                            ⁡                              [                                                                                                    W                        1                        i                                                                                                                        ⋮                                                                                                                          W                                                                              N                            T                                                    ⁢                                                                                                                                i                                                                                            ]                                      ⁢            d                    +                      [                                                                                n                    1                                                                                                ⋮                                                                                                  n                                          N                      R                                                                                            ]                                              Equation        ⁢                                  ⁢        1            
Referring to the Equation 1, H denotes a channel matrix of NR×NT, X is a transmission signal vector of NT×1, and n denotes an additive white Gaussian noise (AWGN) vector of NR×1. The transmission signal vector is X=Wid, which is obtained by multiplying a pre-coding vector mapped to a codebook index (i) selected by the terminal from an NT×L codebook matrix (W=[W1 . . . WL]) composed of L pre-coding vectors, by a transmission data symbol (d).
Wi for maximizing a reception signal strength at the terminal is determined by the following Equation 2 based on the aforementioned Equation 1. And, the terminal transmits a codebook index (i) mapped to Wi to the base station, thereby requesting a pre-coding process.
                    i        =                              argmax                                          k                =                1                            ,                                                          ⁢              …              ⁢                                                          ,              L                                ⁢                      {                                                                            Hw                  k                                                            2                        }                                              Equation        ⁢                                  ⁢        2            
Once the resultant value obtained from the Equation 1 is processed with a signal received by the terminal by using a selected pre-coding vector, the following Equation 3 is obtained.
                                                        (                              Hw                i                            )                        H                    ⁢          r                =                                                                                                                    Hw                    i                                                                    2                            ⁢              d                        +                                                            (                                      Hw                    i                                    )                                H                            ⁢              n                                ⁢                                          ⁢                                          =                                                    λ                i                            ⁢              d                        +                                                            (                                      Hw                    i                                    )                                H                            ⁢              n                                                          Equation        ⁢                                  ⁢        3            
λi, denotes a beam-forming grain obtained by using Wi, and ∂n2 denotes power of an additive white Gaussian noise (AWGN) component. In this case, a Signal-to-Noise Ratio (SNR) can be expressed as the following Equation 4.
                    γ        =                                                                                              Hw                  i                                                            2                                                                      n                                            2                                =                                    λ              i                                      σ              n              2                                                          Equation        ⁢                                  ⁢        4            
Next, will be explained the conventional closed loop MIMO system based on a cooperative transmission method. In this case, B base stations each having NT and K terminals each having NR participate in a cooperative transmission method. Here, N=BNT transmission antennas and M=KNR reception antennas operate between the B base stations and the K terminals. And, M×1 reception signal vectors received by the K terminals when the B base stations transmit signals through the channel matrix (M×N) can be expressed as the following Equation 5.
                                                                        r                ~                            =                            ⁢                                                                                          H                      ~                                        ⁢                                          x                      ~                                                        +                                      n                    ~                                                  =                                                                            H                      ~                                        ⁡                                          (                                                                        W                          ~                                                ⁢                                                  d                          ~                                                                    )                                                        +                                      n                    ~                                                                                                                          =                            ⁢                                                                                          [                                                                                                                                                                  H                                ~                                                            1                                                                                                                                                            ⋮                                                                                                                                                                                              H                                ~                                                            K                                                                                                                          ]                                        ⁡                                          [                                                                                                                                                                  w                                ~                                                            1                                                                                                            …                                                                                                                                              w                                ~                                                            K                                                                                                                          ]                                                        ⁢                                      d                    ~                                                  +                                  n                  ~                                                                                                        =                            ⁢                                                                                          [                                                                                                                                  H                              11                                                                                                            …                                                                                                              H                                                              1                                ⁢                                B                                                                                                                                                                                          ⋮                                                                                ⋱                                                                                ⋮                                                                                                                                                              H                                                              K                                ⁢                                                                                                                                  ⁢                                1                                                                                                                                          …                                                                                                              H                              KB                                                                                                                          ]                                        ⁡                                          [                                                                                                                                  w                                                              i                                11                                                                                                                                          …                                                                                                              w                                                              i                                                                  K                                  ⁢                                                                                                                                          ⁢                                  1                                                                                                                                                                                                                          ⋮                                                                                ⋱                                                                                ⋮                                                                                                                                                              w                                                              i                                                                  1                                  ,                                  B                                                                                                                                                                          …                                                                                                              w                                                              i                                KB                                                                                                                                                        ]                                                        ⁡                                      [                                                                                                                        d                            1                                                                                                                                                ⋮                                                                                                                                                  d                            K                                                                                                                ]                                                  +                                  [                                                                                                              n                          1                                                                                                                                    ⋮                                                                                                                                      n                          K                                                                                                      ]                                                                                        Equation        ⁢                                  ⁢        5            
Here,
{tilde over (H)}
denotes a channel matrix of M×N,
{tilde over (X)}
denotes a transmission signal vector of N×1, and
ñ
denotes an additive white Gaussian noise (AWGN) vector of M×1.
{tilde over (W)}=[{tilde over (W)}1 . . . {tilde over (W)}K]
denotes an M×K pre-coding matrix composed of pre-coding vectors to be used between the B base stations and the K terminals.
{tilde over (H)}m 
is a NR×N channel matrix between the B base stations and the mth terminal. And,
{tilde over (W)}m 
denotes N×1 pre-coding vectors composed of B pre-coding vectors used for the mth terminal by the B base stations.
A channel (Hm n) denotes a NR×NT matrix between the nth base station and the mth terminal. Wi m n denotes NT×1 pre-coding vectors determined to maximize a reception signal strength on the channel (Hm n) between the nth base station and the mth terminal. dm denotes a data symbol to be transmitted to the mth terminal, and nm denotes an additive white Gaussian noise (AWGN) vector of NR×1 of the mth terminal. Wi m n for maximizing a reception signal strength on the channel (Hm n) between the nth base station and the mth terminal is determined by the following Equation 6. And, the terminal transmits a codebook index (im n) mapped to Wi m n to the base station, thereby requesting a pre-coding process.
                              i          mn                =                              argmax                                          k                =                1                            ,                                                          ⁢              …              ⁢                                                          ,              L                                ⁢                      {                                                                                                H                                          mn                      ⁢                                                                                                                            ⁢                                      w                    k                                                                              2                        }                                              Equation        ⁢                                  ⁢        6            
A signal received by the mth terminal can be expressed as the following Equation 7.
                                                                        r                m                            =                            ⁢                                                                                          H                      ~                                        m                                    ⁡                                      (                                                                  W                        ~                                            ⁢                                              d                        ~                                                              )                                                  +                                  n                  m                                                                                                        =                            ⁢                                                                                                                  H                        ~                                            m                                        ⁡                                          [                                                                                                                                                                  w                                ~                                                            1                                                                                                            …                                                                                                                                              w                                ~                                                            K                                                                                                                          ]                                                        ⁢                                      d                    ~                                                  +                                  n                  m                                                                                                        =                            ⁢                                                                                          H                      ~                                        m                                    ⁢                                                            w                      ~                                        m                                    ⁢                                      d                    m                                                  +                                                      ∑                                                                  k                        =                        1                                            ,                                              k                        ≠                        m                                                              K                                    ⁢                                                                                    H                        ~                                            m                                        ⁢                                                                  w                        ~                                            k                                        ⁢                                          d                      k                                                                      +                                  n                  m                                                                                        Equation        ⁢                                  ⁢        7            
When applying 1×NR reception signal process vectors, um=({tilde over (H)}m{tilde over (w)}m)H 
, to the Equation 7 by using a selected pre-coding vector, the following Equation 8 is obtained.
                                                                                          u                  m                                ⁢                                  r                  m                                            =                            ⁢                                                                                                                                                                H                          ~                                                m                                            ⁢                                                                        w                          ~                                                m                                                                                                  2                                +                                  d                  m                                +                                                                            (                                                                                                    H                            ~                                                    m                                                ⁢                                                                              w                            ~                                                    m                                                                    )                                        H                                    ⁢                                      (                                                                                            ∑                                                                                    k                              =                              1                                                        ,                                                          k                              ≠                              m                                                                                K                                                ⁢                                                                                                            H                              ~                                                        m                                                    ⁢                                                                                    w                              ~                                                        k                                                    ⁢                                                      d                            k                                                                                              +                                              n                        m                                                              )                                                                                                                          =                            ⁢                                                                    λ                    m                                    ⁢                                      d                    m                                                  +                                  n                  ′                                                                                        Equation        ⁢                                  ⁢        8            
λm denotes a beam-forming gain obtained through a cooperative transmission method. And, an SINR with consideration of an additive white Gaussian noise (AWGN) signal and a multi-use interference signal can be expressed as the following Equation 9.
                    γ        =                                                                                                                H                    ~                                    m                                ⁢                                                      w                    ~                                    m                                                                    2                                                                                                                  ∑                                                                  k                        =                        1                                            ,                                              k                        ≠                        m                                                              K                                    ⁢                                                                                    H                        ~                                            m                                        ⁢                                                                  w                        ~                                            m                                        ⁢                                          d                      k                                                                      +                                  n                  m                                                                    2                                              Equation        ⁢                                  ⁢        9            
That is, the conventional closed loop MIMO system based on a cooperative transmission method and a non-cooperative transmission method has capacity shown in the Equations 1 and 5. However, it is difficult to derive the Equation 5 in the case of the conventional closed loop MIMO system based on a cooperative transmission method. The reason is because each channel is not independently and identically-distributed in a cooperative transmission method while each channel is independently and identically-distributed in a non-cooperative transmission method. Accordingly, the conventional closed loop MIMO system is not suitable for the cooperative transmission method.
In order to solve this problem, have been required techniques for enhancing a data reception function and signal quality in non-identically distributed channel environments, and capable of effectively applying the techniques to cellular systems.